1. Field of the Invention
The present invention relates to a method for reducing metallic oxides, particularly iron oxides, and to a device for implementing the method.
2. Discussion of the Background
The direct reduction of metallic oxides, particularly ores but also various metallic oxides to be recycled, has developed considerably in recent years.
A method is described in the document LU-60981-A (Socixc3xa9txc3xa9 Anonyme des Minerais) for producing an iron sponge comprising the use of a continuous rotating-hearth reactor with a displacement of the material from the side to the centre, first supplied with coal and then, after the coal has been coked, with iron ore, in pellet form or broken up, preheated to the reaction temperature. Fixed scrapers cause a movement of the coal towards the centre of the furnace and mix the coked coal with the ore as the rotating hearth rotates. After the reaction, the charge is discharged through a central shaft.
One of the disadvantages of the present state of the art is that the volatile constituents of the coal do not take part in the reduction of the metallic oxides. This method does not make it possible to obtain either a high productivity or a high degree of uniformity as regards the temperature and the material of the charge.
The objective of the present invention is to propose a method for reducing metallic oxides making a more efficient use of the reducing capacities of the volatile constituents of a carbonaceous reducing agent.
In conformity with the invention, this objective is attained by a method for the reduction of metallic oxides in a furnace with a ring-shaped rotating hearth in which a carbonaceous reducing agent and metallic oxides are deposited in a strip on a part of the said rotating hearth and are then transported in a roughly helical movement to a discharge device, characterised in that the reducing agent is preheated and mixed with the preheated metallic oxides before or during their deposition on the rotating hearth, in that, in a first reducing stage, the volatile components of the carbonaceous reducing agent (mainly methane and hydrogen) are used to initiate the reduction of the metallic oxides and in that, in a second reducing stage, carbon monoxide is used.
Unlike the methods in the present state of the art, the method according to the invention uses a part of the volatile constituents of the carbonaceous reducing agent, particularly methane and hydrogen, for their reducing capacity.
The method according to the invention makes it possible to increase the reaction rates by a mixing of the metallic oxides and the carbonaceous reducing agent by efficient use of the reducing capacities of the volatile constituents of the carbonaceous reducing agent by their forced passage through the preheated mixture that forms the furnace charge.
One of the advantages of this method lies in the fact that the volatile components, i.e. the distillation gases from the carbonaceous reducing agent, are used in a first stage to reduce the metallic oxides, whereas in known methods these gases are burnt and are used to heat the solid materials.
The metallic oxides are therefore reduced in two stages or by at least two different chemical reactions.
The first reducing stage is carried out using the hydrogen and/or methane released during the heating of the carbonaceous reducing agent. The reaction kinetics of these reactions are more favourable than those of carbon monoxide at temperatures below 900xc2x0 C. The aforesaid volatile constituents are progressively released and make contact with the metallic oxides deposited on the furnace hearth under operating conditions, particularly as regards the reaction temperature, such that they participate in the reduction of the said oxides.
The metallic oxides and the reducing gases released make contact at temperatures as high as possible, but without upsetting the progress of the reduction process. The carbonaceous reducing agent is preferably preheated to a temperature up to 200xc2x0 C., while the metallic oxides are preferably preheated to a temperature up to 850xc2x0 C.
The two constituents are preferably preheated by means of heat recovered from the combustion gases discharged from the furnace into heat exchangers.
These operating conditions lead to an increase in the production capacity per unit surface area and to a reduction in the quantity of carbon dioxide discharged into the atmosphere per unit quantity of the reduced metallic oxides obtained.
This method also has the advantage of discharging less dust outside the furnace thanks to a control over the speed of these gases while keeping the volume of the furnace to a minimum. The metallic sponge obtained has, in bulk, a better homogeneity in the degree of reduction than products resulting from known techniques.
An excess of at least 10% of carbonaceous reducing agent is preferably used, this excess being defined with respect to the theoretical quantity necessary for the reduction of the oxides.
According to a particular form of execution, a method is proposed for the direct reduction of metallic oxides in a rotating-hearth furnace, in which, on a part called the charging zone of the hearth over a certain width of the ring, which depends on the diameter and the capacity of the furnace, a charge consisting of several layers is deposited. These layers may be deposited simultaneously or successively.
The concentrations of metallic oxides and carbonaceous reducing agent in the layers may be different. Preferably, the concentration of metallic oxides in the upper layers is greater than the concentration of metallic oxides in the lower layers. The lower layers consequently contain an excess of carbonaceous reducing agent. The concentration of carbonaceous reducing agent in the upper layers is therefore less than that in the lower layers. In such a case, there is a kind of gradient in the concentration of metallic oxides, a concentration that increases from the hearth in the direction of the upper surface of the charge. A greater quantity of volatile constituents is therefore released in the deep layers and these gases diffuse through the layers towards the upper surface of the charge, where these volatile constituents encounter a higher concentration of metallic oxides. Since the temperature of the lower layers is lower than the temperature of the upper layers, the volatile constituents of the carbonaceous reducing agent are progressively released in the lower layers and, during their diffusion towards the upper surface, encounter very hot metallic oxides. In fact, the upper layers are hotter than the lower layers, firstly because the upper layers contain a higher concentration of metallic oxides preheated to higher temperatures than the carbonaceous reducing agent and secondly because these layers are in contact with the furnace atmosphere. The volatile constituents therefore participate more effectively in the reduction of the metallic oxides.
Advantageously, the concentration of carbonaceous reducing agent in the lower layer lies between the theoretical concentration necessary for the complete reduction of the metallic oxides and a concentration of 100% by weight, preferably between 30% and 70% by weight and, in particular, preferably between 35% and 60% by weight.
The concentration of carbonaceous reducing agent in the upper layer is preferably less than 25% by weight, and, in particular, is preferably less than 16% by weight.
According to an advantageous form of execution, the charge is heated inside the furnace up to a temperature of 900-1250xc2x0 C. and preferably 1050-1150xc2x0 C.
It is worthwhile using the metallic oxides at a feed temperature as high as possible while avoiding the agglomeration of the metallic oxides.
Advantageously, the mixture of carbonaceous reducing agent and metallic oxides, or the charge, is turned over and progressively mixed during its residence inside the furnace.
According to another preferred form of execution, the surface of the charge is shaped by forming furrows or hummocks on it to promote heat exchange between the upper part of the furnace and the charge through an increase in the efficacy of the radiation from the furnace and through an increase in the surface area for heat exchange with the furnace atmosphere.
The slope of the furrows or hummocks normally lies between 40xc2x0 and 65xc2x0.
Preferably, a sawtooth-shaped surface is created on the surface of the charge.
According to a preferred form of execution, the charge or the mixture is charged on to an inner part of the ring-shaped hearth and is transferred in a roughly helical movement towards the outer part of the hearth and, after the reaction, it is discharged through the outer part of the ring.
The mixture is generally discharged after four or more revolutions.
The layer or layers of the mixture of carbonaceous reducing agent and metallic oxides is/are preferably deposited on a part corresponding to xc2xc or less of the width of the ring.
During its residence inside the furnace, the bulk density of the charge decreases, i.e. its volume increases. The flow properties of the charge vary and, in particular, the angle of repose increases, i.e. the slope of the hummocks or furrows may become increasingly steep with the progress of the charge inside the rotating-hearth furnace.
According to a preferred form of execution, as the charge is transported from the central part of the rotating hearth towards the outer part, the width occupied by the strip progressively varies during the process. The increase in bulk volume of the charge during the process is largely compensated by an increase in the slope of the sawtooth-shaped surface and in the width of the strip.
A post-combustion of the gases released during the reduction is preferably achieved in the inner part of the furnace ring.
Advantageously, the discharge of the gases and the movement of the charge inside the furnace take place radially in opposite directions.
The reducing agent and the metallic oxides are commonly preheated by means of the heat recovered from the combustion gases and the post-combustion gases.
It appears advantageous to mix some lime with the metallic oxides and/or with the carbonaceous reducing agent, firstly because the said lime acts as a catalyst for the reaction and secondly because it prevents phenomena of adhesion in the metallic sponges. In addition, the lime generally contributes to desulphurisation of pig iron and to the formation of a more fluid slag or clinker.
In a particular application, the layer consisting of the mixture of metallic oxides and carbonaceous reducing agent is formed by a layer of pellets incorporating these constituents.
The term xe2x80x9cmetallic oxidesxe2x80x9d embraces both metallic ores, particularly iron ore, and metallic oxides to be recycled originating from iron and steel-making processes and from foundries, for example from blast furnaces, steel plants, electric furnaces or rolling mills, as well as a mixture of these sources of oxides with coke fines or with coal, if necessary in the form of pellets.
The term xe2x80x9ccarbonaceous reducing agentxe2x80x9d is understood to mean any carbonaceous material in solid or liquid form, for example coal, lignite and petroleum derivatives. In general, the reducing agent is coal having a concentration of volatile constituents as high as possible in the context of the process, preferably having a concentration of volatile constituents above 15%.
According to another aspect of the invention, a rotating-hearth furnace for the reduction of metallic oxides is also proposed, the said furnace comprising a ring-shaped rotating hearth subdivided into:
a charging zone,
at least one intermediate zone adjacent to the charging zone,
a discharge zone adjacent to the intermediate zone,
the charging zone comprising a device for depositing, on a strip of the rotating hearth, a charge comprising one or more layers of a mixture of metallic oxides and a reducing agent,
the intermediate zone and possibly the charging zone of the furnace comprising a device for progressively stirring an upper part and an underlying part of the charge while displacing the charge radially as the hearth rotates,
the discharge zone comprising a discharging device enabling the metallised charge to be discharged at one or more discharge points.
The furnace comprises a device for creating on the surface of the deposited layer furrows or hummocks, so as to obtain an essentially sawtooth-shaped surface.
The device for depositing one or more layers of a mixture of metallic oxides and a reducing agent may possibly comprise an apparatus for mixing, while hot, the carbonaceous reducing agent and the metallic oxides before, after or during the deposition of the layers.
According to a particular form of execution, the discharge device comprises a worm conveyor or a deflector. In the case where a deflector is used to carry out the discharge from the furnace, the width of the furnace ring may be greater than when using a worm conveyor, because, in view of the high temperatures prevailing inside the furnace, the worm drive, beyond a certain length, is mechanically too heavily loaded.
The furnace advantageously comprises equipment for stirring comprising rabbles provided with blades arranged like the teeth of a rabble, the said rabbles being fixed and arranged radially in the furnace.
The said rabbles comprise blades penetrating the layer while displacing the mixture radially towards the discharge side of the ring.
The blades are generally offset, i.e. arranged in a slightly staggered fashion with respect to the furrows or hummocks formed by the blades of the preceding rabble, so as to remove or level off one side of each furrow and thus form a new furrow.
According to a preferred form of execution, equipment is provided making it possible, by a first action, to bring down the peaks of the saw teeth, which are the hottest part of the furrows, into the hollows of the furrows and, by a second action, to take one face of each saw tooth on to a face of the adjacent saw tooth so as to cover the material brought by the first action.
The working angle of the blades preferably lies between 20xc2x0 and 30xc2x0 with respect to the tangent to the furrows. The working angle of the blades may at any time be adapted in order to reverse the direction of the radial displacement of the charge and in order to increase its residence time inside the furnace.
The blades are preferably shaped so as to turn the charge over.
The furnace advantageously comprises burners installed in the outer walls of the mobile-hearth furnace and/or in the outer ring of the roof in order to maintain the furnace at a temperature of the order of 1200 to 1550xc2x0 C., preferably of the order of 1400xc2x0 C.